Semiconductor switches such as, for example, MOSFET or IGBT, are in widespread use as electronic switches for switching loads. Examples of these are half bridge or bridge circuits, in particular bridge circuits for driving electric motors, or switch-mode power supplies in which there is present a switch regulating the power consumption that is switched in series with an inductive storage element.
In the case of the switching operations, there is firstly the aim of achieving as short as possible a turn-on time, which denotes time duration of a transition from the completely off state to the completely on state of the semiconductor switch, and as short as possible a turn-off time, which denotes the time duration of a transition from the completely on state to the completely off state of the semiconductor switch, in order to minimize switching losses that occur. Secondly, however, the switching edges that occur in the load current or the voltages present across the load and the semiconductor switch are to be suitably flattened off in order to limit the frequency spectrum of the electromagnetic stray radiation. This applies, in particular to switching applications in which the voltage is not shielded by electromagnetic shielding measures such as a metal cage, for example. The particular aim in this case is edge durations of between 50 ns and 100 ns. The frequencies of the stray radiations occurring in this case are then predominantly below 30 MHz, and therefore propagate in a conductive fashion. Such stray radiations can be filtered out by means of filter circuits such that there is no need for expensive mechanical shielding measures.
DE 197 25 837 C2 discloses a drive circuit for an MOS transistor that provides a drive signal for the MOS transistor that has three sections, the drive signal being slowed down during a second section by comparison with a first and third section. Use is also made in this case of the fact that up to a specific value of the drive signal that depends on the component—that is to say the gate-source voltage for MOSFET—the MOS transistor is in the off state and so the drive signal can rise rapidly up to this value without this resulting in a change in the switching state, and therefore in a change in the load path voltage or in the load current. During an adjoining second phase, the component goes over from the off state to the on state, and so the rise in the drive signal is slowed down during this phase in order to reduce the steepness of a switching edge occurring during the second phase. During a subsequent third section of the drive phase, the component is already on, and so the drive signal can rise rapidly during this phase up to its maximum value. Overall, it is possible by means of such a three-stage drive to optimize the switching behavior with reference to the steepness of the switching edges and to switching speed.
In order to carry out such a three-stage drive method, it is proposed in DE 198 55 604 C1 for the purpose of turning on a power MOSFET to supply a first charging current to the drive electrode of the MOSFET during a first drive phase until the drain current of said MOSFET exceeds a current threshold value, and to deliver a second, smaller charging current to the drive electrode during a subsequent second phase until the drain voltage of the MOSFET falls below a prescribed threshold value that signals the turning on of the MOSFET. During a subsequent third phase, the first, larger charging current is supplied to the drive electrode of the MOSFET once again for a prescribed time duration. The method is performed in the reverse sequence when the MOSFET is being turned off.
It is common to the known methods that the criteria for a transition from one drive phase to a next drive phase are tuned permanently to the respective component such that a drive circuit that is tuned for driving a specific type of semiconductor switch can no longer be used for another type which has, for example, another threshold voltage. Even manufacturing fluctuations in the component parameters of a semiconductor switch can have the result that the prescribed thresholds for the transition to another drive phase no longer lead to an optimum drive operation of the semi-conductor switch.